14 
It is known, that for small depths, the period of the backwash T, the distance 4 of 
two successive crests of waves, and the average depth of sea between the two crests p are 
related as follows where g signifies the acceleration of gravity: 
27r À 2 P 
T^ En Coth = 
  
This relation shows that if T is known, it is possible to deduce p from the measurement 
of 4. But 4 can be measured on the photograph and it is possible to determine T by 
noting the time of the breaking of the crests on the shore, or better still, by counting 
the number of crests passing by in a certain time from a boat anchored near the shore. 
This method was tried at the coast west of Cotentin, and it was possible to determine 
depths of about 0 to 5 meters with an accuracy of 5 0/9, 
(d) The use of photogrammetry to study thunder cenires. 
The contrcl of experiments on artificial modification of thunder centres 
requires an objective knowledge of situation, dimension, and speed of cer- 
tain clouds. 
To determine those elements, we have at our disposal within a certain region several 
stations connected by telephone, and when an interesting storm situation turns up, photo- 
graphs are taken simultaneously from these stations. These are localized in relation to 
geodetic points indispensably registered on the photograph. The plotting is made accor- 
ding to graphical and simple numerical methods. In this way there is obtained the plani- 
metric position, height and size of the thunder centre and, by repeating the operation 
after a given time, its speed. 
The cameras are of short focal length, 2,8 to 5 cm; some stations have objectives of 9 
and 13,5 cm. The size of the plates are 25x36 mm. Red filters are mostly used. For very 
distant clouds (300 km), infrared emulsions and filters are used and sometimes polarized 
filters combined with the usual ones. 
An accuracy of 1:1.000 was obtained, which was considered enough, as a knowledge of 
the height of a cloud situated at 10.000 meters with an accuracy of 100 meters is sufficient. 
The method has been used since 1948 especially in the Pyrenees and in particular to 
study hail-storms. ; 
(e) Photogrammetry in glaciology. 
To study the influence of the variation of volume of glaciers on the level 
of the water in the barrages they provide, M. Poivilliers with the aid of 
the »Institut Géographique National» undertook for »l'Electricité de 
France» the plotting of photographs of certain regions of the Mont Blanc, 
taken at different seasons. The work is in progress and a report of it will 
probably be given at the congress in Washington. 
(f) Photogrammetry in astronomy. 
Modern astronomy commonly uses measurements made on photographs 
to study such phenomena as the height and speed of meteors, the movement 
of the stars, the tracks of comets and asteroids, the distribution of the 
stars in the atmosphere, the distance and movement of the galaxies etc. 
Stereoscopic measurements (Mr. Rosch) are commonly used today, which 
facilitate accurate determinations of certain astronomic quantities, such 
as the magnitude of the stars, the actual wave-length, the proper movements 
of the stars, the colour of nebulae etc. 
There was no particular problem when taking the photographs. The 
measurements were made in a Pulfrich stereocomparator. 
A complete account of these methods was given at the congress in the 
Hague 1948. 
Sweden: 
The chief development of special photogrammetry in Sweden is found in its 
application to different new domains. However, some new methods are used. 
Tol 
numbe 
camera 
of car 
measul 
numbe 
tapes 
were S 
determ 
the ta] 
determ 
the po: 
Stereos 
of the 
interva 
Mult 
size. C 
be rigc 
In o 
the sea 
carried 
Stereos 
Two s 
tion of 
At m 
difficul 
Special 
The 
the foc 
toe. TI 
The pe 
120 cn 
with tk 
Prob 
Proc 
thickne 
tor (P 
success 
Resu 
ness in 
1 To 
treatm 
Pro 
ked ou 
couple: 
the vo 
conditi 
compo 
Res 
2. Wo 
Pro 
picture 
was m 
tate th 
on it a 
3. Det 
Pro 
be fou 
a chall